1. Field of the Invention
The present invention relates to the processing of positioning signals, such as signals received from the NAVSTAR Global Positioning System (GPS). In particular, the present invention relates to detecting positioning signals, by first using a less sensitive, more robust technique, followed with further processing using a more sensitive technique.
2. Discussion of the Related Art
Algorithms for the detecting positioning signals, such as GPS signals, differ in their sensitivity, robustness, ease of implementation, and computational resources requirements. Often, sensitive algorithms are computationally more demanding and less robust than less sensitive algorithms.
Two well-known algorithms for signal detection are coherent integration and non-coherent integration. In coherent integration, the received positioning signal is multiplied variously with locally generated replicas of the expected satellite signal of different code phases and carrier frequencies, and each product is integrated for a time period equal to the signal capture time. Following this integration step, the magnitude of each integrated product is computed and, after further processing that may include interpolation, the magnitudes are each compared to a threshold to ascertain whether the positioning signal to be detected is present. Non-coherent integration is similar to coherent integration, except that the magnitudes or square magnitudes are computed at regular subintervals of the signal capture and the results are then summed, processed, and finally compared to a threshold value.
In both the coherent and the non-coherent integration algorithms, multiple trial code phases and frequencies are tested. In coherent integration, the trial frequencies (the “frequency bins”) are spaced by       1    T    ,where T is the interval of coherent integration. Compared to non-coherent integration, coherent integration has a higher sensitivity, at the expense of a lesser robustness and a higher computational requirement. Coherent integration methods have been developed that trade robustness for less computational complexity. Coherent integration may also search over additional parameters such as oscillator phase acceleration.
In certain environments, such as in urban locations, the positioning signals may be attenuated because of propagation through walls, reflections off of buildings and other signal distortions. Typically, because of the spatial distribution of the positioning signal sources, the positioning signals from each of the sources will be subject to different amounts of attenuation and reflection. As a result, it is rare that two positioning signals are attenuated by the same amount. Indeed, the signal powers of the received sources typically have a distribution in which there is considerable difference between the strongest positioning signal and the other signals.
Numerous techniques for GPS signal detection are disclosed in the following U.S. patent applications:                1. Signal Acquisition using Data Bit Information (Ser. No. 09/888,228 filed Jun. 22, 2001, hereinafter referred to as “Application 228”),        2. Synthesizing Coherent Correlation Sums at One or Multiple Carrier Frequencies Using Correlation Sums Calculated at a Coarse Set of Frequencies (Ser. No. 09/888,227 filed Jun. 22, 2001, hereafter referred to as “Application 227”),        3. Extracting Fine-Tuned Estimates from Correlation Functions Evaluated at Limited Number of Values (Ser. No. 09/888,338 filed Jun. 22, 2001, hereafter referred to as “Application 338”),        4. Determining the Spatio-Temporal and Kinematic Parameters of a Signal Receiver and its Clock by Information Fusion (Ser. No. 09/888,229 filed Jun. 22, 2001, hereafter referred to as “Application 229”),        5. Determining Location Information Using Sampled Data Containing LocationDetermining Signals And Noise (Ser. No. 09/888,337 filed Jun. 22, 2001, hereafter referred to as “Application 337”),        6. Method for optimal search scheduling in satellite acquisition (Ser. No. 10/126,853, filed on Apr. 19, 2002, hereafter referred to as “Application 853”),        7. System and method to estimate the location of a receiver in a multi path environment (Ser. No. 10/237,556, filed on Sep. 6, 2002, hereafter referred to as “Application 556”),        8. System and method to estimate the location of a receiver (Ser. No. 10/237,557 filed on Sep. 6, 2002, hereafter referred to as “Application 557”), and        9. Multifunction device with positioning system and shared processor (Ser. No. 10/286,360 filed on Nov. 1, 2002, hereafter referred to as “Application 360”).        
The above patent applications are incorporated by reference herein in their entireties.